Computed tomography image generation apparatus

ABSTRACT

The invention relates to a CT image generation apparatus ( 10 ) for generating an image of a head. Transformations of a first CT image of the head are determined for different measured projection groups, wherein for a measured projection group a transformation is determined such that a degree of similarity between the measured projection group and a calculated projection group is increased, wherein the calculated projection group is calculated by transforming the first CT image in accordance with the transformation to be determined and by forward projecting the transformed first CT image. A motion corrected three-dimensional second CT image is reconstructed based on the measured projections and the transformations determined for the different measured projection groups. This allows providing a high quality CT image of the head, even if a patient cannot stop moving the head in case of, for instance, stroke.

FIELD OF THE INVENTION

The invention relates to a computed tomography image generationapparatus and method for generating an image of a human head. Theinvention relates further to a computer program for controlling thecomputed tomography image generation apparatus.

BACKGROUND OF THE INVENTION

The article “Motion and Positional Error Correction for Cone Beam3D-Reconstruction with Mobile C-Arms” by C. Bodensteiner et al., MedicalImage Computing and Computer-Assisted Intervention, volume 4791, pages177-185 (2007) discloses a mobile C-arm device for acquiring computedtomography images. The C-arm device is configured to perform iterativethree-dimensional reconstruction and two-dimensional/three-dimensionalregistration algorithms for correcting projection data inconsistencies,wherein the projection data inconsistencies are caused by positioningerrors of the C-arm device.

Computed tomography image generation apparatuses for generating an imageof a human head comprise a reconstruction unit for reconstructing theimage based on acquired projections. The projections depend onradiation, which has traversed the head and which is emitted by aradiation source moving around the head along, for instance, a circularor helical trajectory. Patients like acute stroke patients may not beable to avoid motion during the acquisition of the projections such thatthe reconstructed image of the head may comprise motion artifacts.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a computedtomography image generation apparatus and method for generating an imageof a human head which has less motion artifacts. It is a further objectof the present invention to provide a computer program for controllingthe computed tomography image generation apparatus in accordance withthe computed tomography image generation method.

In a first aspect of the present invention a computed tomography imagegeneration apparatus for generating an image of a human head ispresented, wherein the computed tomography image generation apparatuscomprises:

-   -   a projections providing unit for providing measured        two-dimensional projections of the head, wherein the measured        projections have been measured at different times while a        radiation source, which emits radiation for traversing the head,        has been moved around the head and wherein the measured        projections have been generated based on the radiation after        having traversed the head,    -   a reconstruction unit for reconstructing a three-dimensional        first computed tomography image of the head based on the        provided measured projections,    -   a transformation determination unit for determining        three-dimensional transformations of the first computed        tomography image of the head for different measured projection        groups, wherein a measured projection group comprises one or        several measured projections, wherein the transformation        determination unit is adapted to determine for a certain        measured projection group a transformation such that a degree of        similarity between the certain measured projection group and a        calculated projection group is increased, wherein the calculated        projection group corresponds to the certain measured projection        group and is calculated by transforming the first computed        tomography image in accordance with the transformation to be        determined and by forward projecting the transformed first        computed tomography image,

wherein the reconstruction unit is adapted to reconstruct a motioncorrected three-dimensional second computed tomography image based onthe measured projections and the transformations determined for thedifferent measured projection groups,

wherein the computed tomography image generation apparatus comprises anexamination zone in which the human head is arrangeable, wherein thereconstruction unit is adapted to reconstruct the first computedtomography image and the second computed tomography image such that theyshow the examination zone and to, for generating the second computedtomography image, perform the motion correction for a first part of theexamination zone and to perform the motion correction not for a secondpart of the examination zone, wherein the first part of the examinationzone includes the human head.

Since the measured projections have been measured at different times andsince for each measured projection group, which comprises one or severalmeasured projections, a transformation of the first computed tomographyimage of the head is determined such that a degree of similarity betweenthe respective measured projection group and the correspondingcalculated projection group is increased, especially optimized, whereinthe corresponding calculated projection group is determined bytransforming the first computed tomography image in accordance with thetransformation to be determined and by forward projecting thetransformed first computed tomography image, for different timestransformations of the first computed tomography image are determined,which transform the first computed tomography image such that it is inaccordance with the respective measured projection group. Thetransformations determined for the different measured projection groupsand hence for the different times therefore accurately describe themotion of the head and can be used for reconstructing a motion correctedthree-dimensional second computed tomography image having only fewmotion artifacts or no motion artifacts at all.

A measured projection group can comprise a single measured projection orseveral measured projections. If a measured projection group comprisesseveral measured projections, these measured projections arepreferentially temporally adjacent. These measured projections of a samemeasured projection group are thereby also angularly adjacent withrespect to a rotational angle describing the rotation of the radiationsource around the head, i.e. a measured projection group comprisingseveral measured projections can therefore also be regarded ascomprising an angular subset of the measured projections.

The projections providing unit can be a storing unit for storingmeasured projections and for providing the stored measured projections.The projections providing unit can also be a receiving unit forreceiving the measured projections from a projections acquisition deviceand for providing the received measured projections. Moreover, theprojections providing unit can also be the projections acquisitiondevice itself. The projections acquisition device may comprise aradiation source for emitting radiation, a detection device fordetecting the radiation after having traversed the human head and forgenerating the measured projections based on the traversed radiation andmeans for moving the radiation source and the human head relative toeach other for generating the measured projections in differentacquisition directions.

The projections providing unit is preferentially adapted to providemeasured projections which have been measured by using a step-and-shootacquisition scheme or a helical acquisition scheme. This allowsgenerating the first and second computed tomography images such thatthey cover a relatively large part of the head, especially the entirehead, by using a detection unit having a relatively small extensionparallel to a rotational axis of the rotational movement of theradiation source around the head.

A calculated projection corresponds to a measured projection, if theyhave been calculated and measured, respectively, in the same acquisitiongeometry, wherein the acquisition geometry defines the geometry of therays of the generated, real radiation used for generating the measuredprojection and the geometry of the rays of the simulated radiation usedfor generating the calculated projection. If a calculated projectiongroup and a measured projection group correspond to each other, theyhave the same number of projections and each projection of the measuredprojection group has a corresponding projection in the calculatedprojection group.

The transformation determination unit is preferentially adapted todetermine rigid transformations for transforming the first computedtomography image. The transformation determination unit can be adaptedto determine a transformation for a measured projection groupiteratively. Moreover, in an embodiment the transformation determinationunit is adapted to use a gradient angle difference for determining thedegree of similarity. In particular, the transformation determinationunit is adapted to use a normalized gradient angle difference fordetermining the degree of similarity. However, the transformationdetermination unit can also be adapted to use other similarity measuresfor determining the degree of similarity like a sum of squareddifferences, a gradient correlation measure, a pattern intensitymeasure, et cetera.

The reconstruction unit can be adapted to reduce motion artifacts in thefirst computed tomography image by using overscan weighting. By usingoverscan weighting a low level motion artifacts reduction can beprovided such that for determining the transformations a first computedtomography image can be used having an improved image quality. This canlead to an improved determination of the transformations and hence to animproved motion correction when reconstructing the second computedtomography image.

In an embodiment the transformation determination unit is adapted suchthat each measured projection group comprises a single measuredprojection. If each measured projection group comprises a singlemeasured projection only, for each single measured projection atransformation is determined such that the motion of the head can becharacterized by the determined sequence of transformations with arelatively high temporal resolution.

In a further embodiment the transformation determination unit is adaptedsuch that each measured projection group comprises several measuredprojections. Thus, in an embodiment several measured projections areused for determining a respective transformation. This can lead to amore accurate determination of the transformations for the differentmeasured projection groups, which describe the motion of the head, andhence to a further improved correction of the motion artifacts in thesecond computed tomography image which is reconstructed based on themeasured projections and the determined transformations.

The transformation determination unit can be adapted to filter thedetermined transformations. For instance, the transformationdetermination unit can be adapted to use a smoothing filter, which mightbe a median filter or another filter, for smoothing the determinedtransformations.

In an embodiment the transformation determination unit is adapted todetermine transformations based on the degree of similarity and theforward projection not for all times, at which projections have beenmeasured, and to determine transformations for times, at whichtransformations have not been determined based on the degree ofsimilarity and the forward projection, by interpolation. This has theadvantage that the computational efforts for determining thetransformations can be reduced. Moreover, the transformationdetermination unit can be adapted to determine an outlier in thedetermined transformations and to remove the determined outlier from thedetermined transformations. By removing the determined outlier from thedetermined transformations it can be ensured that the outlier is notused for motion correction while reconstructing the second computedtomography image. This can further improve the quality of thereconstructed second computed tomography image. The removed outliertransformation may be replaced by a transformation which has beendetermined by interpolation, wherein an interpolation algorithm may beapplied to temporally neighboring transformations. The replacingtransformation may also be determined in another way, for instance, bydetermining the median of the temporally neighboring transformations.For determining the replacing transformation one or severaltransformations corresponding to earlier times and one or severaltransformations corresponding to later times may be used.

The interpolation and/or filtering algorithms are preferentially appliedto parameters of the transformations along the temporal axis. Forinstance, if the transformations include three translational parametersdefining a translation, the interpolation and/or filtering algorithmsare applied to corresponding translational parameters of differenttransformations, i.e. which have been determined for different times.The transformations preferentially only include translations androtations. However, they can also include a deformation.

The computed tomography image generation apparatus comprises anexamination zone in which the human head is arrangeable, wherein thereconstruction unit is adapted to reconstruct the first computedtomography image and the second computed tomography image such that theyshow the examination zone, wherein in an embodiment the reconstructionunit is further adapted to, for generating the second image computedtomography image, perform the motion correction for a first part of theexamination zone and to perform the motion correction not for a secondpart of the examination zone, wherein the first part of the examinationzone includes the human head. This can reduce the computational effortsfor reconstructing the second computed tomography image. The first partand the second part of the examination zone can be predefined. Forinstance, the part of the examination zone, which is known to include apatient table and an area below the patient table, can be predefined asbeing the second part, whereas the first part can be the remaining partof the examination zone. It is also possible that the reconstructionunit determine the first part of the examination zone including thehuman head based on the first computed tomography image by using, forinstance, known segmentation algorithms for segmenting the human head inthe first computed tomography image. The first part could then includethe segmented human head and a predefined margin surrounding thesegmented human head. The predefined margin can ensure that really thecomplete human head is within the first part, even if the human headmoves.

In an embodiment the reconstruction unit is adapted to divide themeasured projections into several sets of measured projections, whereineach set includes temporally adjacent measured projections, and toreconstruct several three-dimensional first computed tomography imagesof the head based on the several sets of measured projections, wherein arespective first computed tomography image is reconstructed based on arespective set of measured projections. In this embodiment thetransformation determination unit is adapted to determine for each firstcomputed tomography image a set of three-dimensional transformations ofthe respective first computed tomography image of the head for differentmeasured projection groups, wherein a measured projection groupcomprises one or several measured projections of the respective set ofmeasured projections, wherein the transformation determination unit isadapted to determine for a certain measured projection group of therespective set of measured projections a transformation such that adegree of similarity between the certain measured projection group and acalculated projection group is increased, wherein the calculatedprojection group corresponds to the certain measured projection groupand is calculated by transforming the respective first computedtomography image in accordance with the transformation to be determinedand by forward projecting the transformed respective first computedtomography image. The reconstruction unit can then be adapted toreconstruct a motion corrected three-dimensional second computedtomography image based on the measured projections and the several setsof transformations determined for the different measured projectiongroups of the several sets of measured projections. Since not allmeasured projections are used for reconstructing a respective firstcomputed tomography image, the measured projections used forreconstructing the respective first computed tomography image cover arelatively small time period such that the respective reconstructedfirst computed tomography image comprises less motion artifacts.Moreover, since these first computed tomography images showing lessmotion artifacts are used for determining the transformations, thequality of the transformations and hence of the finally reconstructedmotion corrected second computed tomography image can be improved. Thetransformation determination unit can be adapted to determine a furthertransformation transforming a transformed first computed tomographyimage, which has been reconstructed based on a set of measuredprojections and which has then be transformed, such that it correspondsto another transformed first computed tomography image, which has beenreconstructed based on another set of measured projections and which hasthen be transformed, wherein the reconstruction unit can be adapted toreconstruct the motion corrected three-dimensional second computedtomography image also based on the further transformation. The furthertransformation is preferentially a registration transformationregistering the transformed first computed tomography images onto eachother. Preferentially, several further transformations are determinedfor several pairs of transformed reconstructed first computed tomographyimages, which have been reconstructed based on neighboring measuredprojection groups which correspond to different sets of measuredprojections. By using these further transformations the quality of thefinally reconstructed second computed tomography image can be furtherimproved.

The reconstruction unit can be adapted to divide the measuredprojections into several sets of measured projections such that at leastone set of measured projections does not fulfill the completenesscriterion for computed tomography reconstruction. The completenesscriterion for computed tomography reconstruction defines that projectionrays covering an angular range of 180 degrees should be available forevery voxel of the respective first computed tomography image to bereconstructed. Since at least one set of the measured projections doesnot fulfil this completeness criterion, the measured projections of thisset may correspond to an angular range of, for instance, 20 degrees, 30degrees or 40 degrees of a rotation of the radiation source around thehead. In an embodiment the reconstructing unit is adapted to divide themeasured projections into the several sets of measured projections suchthat each set of measured projections does not fulfill the completenesscriterion. For instance, the measured projections can be divided intosets of measured projections, wherein a first set corresponds to anangular range from 0 degrees to 20 degrees, a second set corresponds toan angular range from 21 degrees to 40 degrees, a third set correspondsto an angular range from 41 degrees to 60 degrees et cetera, whereinthese angular ranges refer to the rotational position of the radiationsource while rotating around the head.

In an embodiment the reconstruction unit and the transformationdetermination unit are adapted such that, after the second computedtomography image has been reconstructed, in an iteration step thetransformations are determined again based on the second computedtomography image, which then has the function of the first computedtomography image, and the second computed tomography image is againreconstructed based on the newly determined transformations and themeasured projections. This iteration can be repeated, until an abortcriterion is fulfilled. This can lead to a further improved quality ofthe finally generated second computed tomography image.

In a further aspect of the present invention a computed tomography imagegeneration method for generating an image of a human head is presented,wherein the computed tomography image generation method comprises:

-   -   providing measured two-dimensional projections of the head by a        projections providing unit, wherein the measured projections        have been measured at different times while a radiation source,        which emits radiation for traversing the head, has been moved        around the head and wherein the measured projections have been        generated based on the radiation after having traversed the        head,    -   reconstructing a three-dimensional first computed tomography        image of the head based on the provided measured projections by        a reconstruction unit,    -   determining three-dimensional transformations of the first        computed tomography image of the head for different measured        projection groups by a transformation determination unit,        wherein a measured projection group comprises one or several        measured projections, wherein the transformation determination        unit determines for a certain measured projection group a        transformation such that a degree of similarity between the        certain measured projection group and a calculated projection        group is increased, wherein the calculated projection group        corresponds to the certain measured projection group and is        calculated by transforming the first computed tomography image        in accordance with the transformation to be determined and by        forward projecting the transformed first computed tomography        image,

wherein the reconstruction unit reconstructs a motion correctedthree-dimensional second computed tomography image based on the measuredprojections and the transformations determined for the differentmeasured projection groups, wherein the computed tomography imagegeneration apparatus comprises an examination zone in which the humanhead is arrangeable, wherein the reconstruction unit reconstructs thefirst computed tomography image and the second computed tomography imagesuch that they show the examination zone and to, for generating thesecond computed tomography image, perform the motion correction for afirst part of the examination zone and to perform the motion correctionnot for a second part of the examination zone, wherein the first part ofthe examination zone includes the human head.

In another aspect of the present invention a computer program forcontrolling a computed tomography image generation apparatus as definedin claim 1 is presented, the computer program comprising program codemeans for causing the computed tomography image generation apparatus tocarry out the steps of the computed tomography image generation methodas defined in claim 14, when the computer program is run on a computercontrolling the computed tomography image generation apparatus.

It shall be understood that the computed tomography image generationapparatus of claim 1, the computed tomography image generation method ofclaim 14, and the computer program of claim 15 have similar and/oridentical preferred embodiments, in particular, as defined in thedependent claims.

It shall be understood that a preferred embodiment of the presentinvention can also be any combination of the dependent claims or aboveembodiments with the respective independent claim.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings:

FIG. 1 shows schematically and exemplarily an embodiment of a computedtomography image generation apparatus for generating an image of a humanhead,

FIG. 2 illustrates a first computed tomography image,

FIG. 3 illustrates a calculated projection obtained by forwardprojecting the first computed tomography image,

FIG. 4 illustrates a gradient angle difference image, and

FIG. 5 shows a flowchart exemplarily illustrating an embodiment of acomputed tomography image generation method.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows schematically and exemplarily a computed tomography imagegeneration apparatus 10 for generating an image of a human head. Thecomputed tomography image generation apparatus 10 comprises a gantry 1which is capable of rotation about a rotational axis R which extendsparallel to the z direction. A radiation source 2, which is, in thisembodiment, an x-ray tube, is mounted on the gantry 1. The radiationsource 2 is provided with a collimator 3, which forms, in thisembodiment, a conical radiation beam 4 from the radiation generated bythe radiation source 2. The radiation traverses a human head of apatient (not shown) within an examination zone 5 being, in thisembodiment, cylindrical. After having traversed the examination zone 5and hence the human head, the radiation beam 4 is incident on adetection device 6 which comprises a two-dimensional detection surface.The detection device 6 is mounted on the gantry 1.

The computed tomography image generation apparatus 10 comprises twomotors 7, 8. The gantry 1 is driven at a preferably constant butadjustable angular speed by the motor 7. The motor 8 is provided fordisplacing the patient arranged on a patient table in the examinationzone 5 parallel to the direction of the rotational axis R or the z axis.These motors 7, 8 are controlled by a control unit 9, for instance, suchthat the radiation source 2 and the human head within the examinationzone 5 move relative to each other along a helical trajectory. However,it is also possible that the radiation source 2 and the human head moverelatively to each other along another trajectory. For instance, in anembodiment a step-and-shoot acquisition scheme can be used, whereinfirstly the radiation source 2 is moved around the human head along acircular trajectory at a first longitudinal position of the radiationsource 2 with respect to the patient, wherein then the patient table ismoved such that the radiation source 2 is located at anotherlongitudinal position with respect to the patient, wherein at this otherlongitudinal position the radiation source 2 is moved around the humanhead along a circular trajectory. The patient table can then be movedagain, in order to acquire further projections by moving the radiationsource 2 around the human head along a circular trajectory at anotherlongitudinal position of the radiation source 2 with respect to thepatient, and so forth.

During a relative movement of the radiation source 2 and the human headthe detection device 6 generates measured values depending on theradiation incident on the detection surface of the detection device 6.Measured values, which have been generated at a same time while theradiation source 2 was at a same position relative to the human head,form a measured projection. The radiation source 2, the elements formoving the radiation source 2 relative to the human head, in particular,the motors 7, 8 and the gantry 1, and the detection device 6 cantherefore be regarded as being components of a projections providingunit 12 being, in this embodiment, a projection acquisition device 12,for providing measured two-dimensional projections of the human head.

The measured projections are provided to a projection processing device18 comprising a reconstruction unit 13 and a transformationdetermination unit 14, wherein also the projection processing device 18is preferentially controlled by the control unit 9. The reconstructionunit 13 is adapted to reconstruct a three-dimensional first computedtomography image 15 of the human head based on the measured projections.The reconstruction unit 13 can be adapted to use known reconstructionalgorithms like a filtered back projection algorithm or a Radoninversion algorithm for reconstructing the first computed tomographyimage. An example of a first computed tomography image 15 is illustratedin FIG. 2. As can be seen in FIG. 2, the reconstructed first computedtomography image comprises motion artifacts 19. The reconstruction unit13 is preferentially adapted to reduce the motion artifacts 19 in thefirst computed tomography image 15 by using overscan weighting, whichcan also be regarded as redundant ray weighting. The minimum amount ofprojections required by the reconstruction unit 13 for reconstructingthe first computed tomography image corresponds to an acquisition inwhich each voxel of the first computed tomography image to bereconstructed sees the radiation source 2 over an angular range of 180degrees in a parallel beam geometry or over an angular range of 180degrees plus fan angle in a focus centered geometry. Additional rays,i.e. additional projection values, can be used to apply a smoothweighing at the beginning and end of a respective 180 degrees segment or180 degrees plus fan angle segment, respectively, wherein the weightscan decrease within increasing distance from the mid of the respectivesegment. For more details regarding this overscan weighting reference ismade to, for instance, pages 264 to 266 in the book “Computed TomographyPrinciples, Design, Artifacts, and Recent Advances” by Jiang Hsieh,second edition, John Wiley & Sons (2009), which are herewithincorporated by reference.

The transformation determination unit 14 is adapted to determinethree-dimensional transformations of the first computed tomography image15 of the head for different measured projection groups. Generally, ameasured projection group can comprise a single measured projection onlyor several measured projections. In this embodiment each measuredprojection group comprises a single measured projection. Thethree-dimensional transformation determination unit 14 is adapted todetermine for each measured projection group a transformation such thata degree of similarity between the respective measured projection groupand a calculated projection group is increased, wherein the calculatedprojection group corresponds to the respective measured projection groupand is calculated by transforming the first computed tomography image 15in accordance with the transformation to be determined and by forwardprojecting the transformed first computed tomography image 15. That ameasured projection group corresponds to a calculated projection groupmeans that for the forward projection of the transformed first computedtomography image 15 the same acquisition geometry is used as used whenacquiring the respective measured projection group. In this embodimenteach calculated projection group comprises a single calculatedprojection only, wherein a calculated projection 16 is exemplarily shownin FIG. 3.

The transformation determination unit 14 is preferentially adapted todetermine the degree of similarity by applying a similarity measure onthe respective measured projection group and the respectivecorresponding calculated projection group, wherein this similaritymeasure is preferentially the normalized gradient angle difference asdisclosed, for instance, in the article “Evaluation of optimizationmethods for intensity-based 2D-3D registration in x-ray guidedinterventions” by I. M. J. van der Bom et al., SPIE Medical Imaging(2011), which is herewith incorporated by reference. However, also othersimilarity measures can be used like a gradient correlation measure or apattern intensity measure, which are also disclosed in the article by I.M. J. van der Bom, a sum of squared differences, et cetera. A gradientangle difference 17 is schematically and exemplarily illustrated in FIG.4.

The transformation determination unit 14 is preferentially adapted todetermine a transformation for a measured projection group iteratively.In particular, in each iteration step the transformation, which has beendetermined in the previous iteration step, is modified such that thedegree of similarity between the respective measured projection groupand the calculated projection group is increased, wherein theseiteration steps may be performed until an abort criterion is fulfilled.The abort criterion is, for instance, that the degree of similaritybetween the respective measured projection group and the calculatedprojection group is larger than a predefined threshold or that apredefined maximum number of iterations has been reached.

The transformation determination unit 14 is preferentially furtheradapted to filter the determined transformations. In particular, thetransformation determination unit 14 can be adapted to apply a smoothingfilter like a median filter or another smoothing filter to thedetermined transformations. Moreover, the transformation determinationunit 14 can be adapted to determine transformations based on the degreeof similarity and the forward projection not for all times, at whichprojections have been measured, and to determine transformations fortimes, at which transformations have not been determined based on thedegree of similarity and the forward projection, by interpolation. Theinterpolation can be a linear interpolation applied to correspondingparameters of the transformations. Higher orders of interpolation are ofcourse also possible.

The transformations determination unit 14 can also be adapted todetermine an outlier in the determined transformations and to remove thedetermined outlier from the determined transformations. The removedoutlier transformation may be replaced by a transformation which hasbeen determined by interpolation, wherein an interpolation algorithm maybe applied to temporally neighboring transformations. The replacingtransformation may also be determined in another way, for instance, bydetermining the median of the temporally neighboring transformations.For determining the replacing transformation one or severaltransformations corresponding to earlier times and one or severaltransformations corresponding to later times may be used.

The transformations determined for the different measured projectiongroups and hence for different times describe the motion of the head.The determined transformations can therefore be used by thereconstruction unit 13 to reconstruct a motion correctedthree-dimensional second computed tomography image based on the measuredprojections. For reconstructing the motion corrected three-dimensionalsecond computed tomography image known motion-correcting reconstructionalgorithms may be used like the algorithms disclosed in the articles“Theoretical framework for a dynamic cone-beam reconstruction algorithmbased on a dynamic particle model” by P. Grangeat et al., Physics inMedicine and Biology, volume 47, pages 2611 to 2625 (2002),“Compensation of Some Time Dependent Deformations in Tomography” by L.Desbat et al., IEEE Transactions on Medical Imaging, volume 26, number2, pages 261 to 269 (2007), and “Dynamic X-Ray Computed Tomography” byS. Bonnet et al., Proceedings of the IEEE, volume 91, number 10, pages1574 to 1587 (2003), which are herewith incorporated by reference. Oneexemplary way of reconstructing a motion correction three-dimensionalsecond computed tomography image uses the inverse of the estimatedmotion, i.e. of the determined transformations, for compensating themotion as will be illustrated in the following.

If projections p(α, u, v), ramp filtered projections rfp(α, u, v) and areconstructed image f(x, y, z) are assumed, a filtered backprojectionreconstruction can be formulated as

$\begin{matrix}{{f\left( {x,y,z} \right)} = {\beta {\sum\limits_{\alpha}{{f\left( {\alpha,x,y,z} \right)}\mspace{14mu} {with}}}}} & (1) \\{{{f\left( {\alpha,x,y,z} \right)} = {{BP} \cdot {w\left( {\alpha,u,v} \right)} \cdot {{rfp}\left( {\alpha,u,v} \right)}}},} & (2)\end{matrix}$

wherein w(α, u, v) is a known weight that considers redundant rays, β isa scaling constant, α denotes a rotational angle of the radiation source2 and BP is the back projection operator connecting the image space withthe projection domain. In case that M is a transformation matrix, whichpreferentially includes translation and rotation and which is used tooptimally fit the volume to a projection p(α, u, v), following can bedefined:

$\begin{matrix}{{{MIf}\left( {x,y,z} \right)} = {\beta {\sum\limits_{\alpha}{{{MIf}\left( {\alpha,x,y,z} \right)}\mspace{14mu} {with}}}}} & (3) \\{{{MIf}\left( {\alpha,x,y,z} \right)} = {{MIBP} \cdot {w\left( {\alpha,u,v,M} \right)} \cdot {{{rfp}\left( {\alpha,u,v} \right)}.}}} & (4)\end{matrix}$

The weights w are adapted according to the transformation matrix M,because due to the motion a non-equiangular sampling may be generated.MI is the inverse motion operator in the image space which takes theresult after back projecting a single view and compensates the estimatedtranslation and rotation. The motion-corrected second computedtomography image is denoted by Mlf(x, y, z). The use of the inverse ofthe estimated motion, i.e. of the determined transformations, can ofcourse also be formulated in another way.

The reconstructed second computed tomography image is provided to adisplay unit 11 for displaying the generated second computed tomographyimage. Also the first computed tomography image, the measuredprojections and/or the calculated projections may be shown on thedisplay unit 11.

The reconstruction unit 13 and the transformation determination unit 14can be adapted such that the second computed tomography image isiteratively reconstructed, wherein, after the second computed tomographyimage has been reconstructed, in an iteration step the transformationsare determined again based on the second computed tomography image,which then has the function of the first computed tomography image, andthe second computed tomography image is again reconstructed based on thenewly determined transformations and the measured projections. Theiteration can be performed, until an abort criterion is fulfilled. Theabort criterion might be, for instance, that a difference between acurrent second computed tomography image and a previous second computedtomography image is smaller than a predefined threshold, or that amaximum number of iterations has been reached.

In an embodiment the reconstruction unit 13 can be adapted to divide themeasured projections into several sets of measured projections, whereineach set includes temporally adjacent measured projections, and toreconstruct several three-dimensional first computed tomography imagesof the head based on the several sets of measured projections, wherein arespective first computed tomography image is reconstructed based on arespective set of measured projections. In this embodiment thereconstruction unit 13 is adapted to divide the measured projectionsinto several sets of measured projections such that a respective set ofmeasured projections does not fulfill the completeness criterion forcomputed tomography reconstruction. Thus, the reconstruction unit 13 isadapted to reconstruct each first computed tomography image by using anincomplete set of measured projections. Preferentially, thereconstruction unit 13 is adapted to perform this reconstruction of thefirst computed tomography images by using known tomosynthesisreconstruction algorithms, wherein the respective set of measuredprojections might be weighted by using a weighting function.Preferentially, the weighting function is adapted to provide a weightingfactor which decreases towards the edges of the angular range to whichthe measured projections of the respective set correspond. The weightingfunction might be, for instance, a trapezoidal weighting function, aGaussian weighting function or a Hanning weighting function.

The transformation determination unit 14 can be adapted to determine foreach first computed tomography image a set of three-dimensionaltransformations of the respective first computed tomography image of thehead for different measured projection groups, wherein a measuredprojection group comprises one or several measured projections of therespective set of measured projections. Moreover, the transformationdetermination unit 14 can be adapted to determine for a respectivemeasured projection group of the respective set of measured projectionsa transformation such that a degree of similarity between the respectivemeasured projection group and a calculated projection group isincreased, wherein the calculated projection group corresponds to therespective measured projection group and is calculated by transformingthe respective first computed tomography image in accordance with thetransformation to be determined and by forward projecting thetransformed respective first computed tomography image.

The transformation determination unit 14 can be further adapted todetermine further transformations transforming pairs of transformedfirst computed tomography images, which have been reconstructed based onneighboring sets of measured projections and which have then betransformed by using neighboring measured projection groups which belongto the neighboring sets of measured projections, i.e. one of thesemeasured projection groups belongs to one of these sets of projectionsand the other of these measured projection groups belongs to the otherof the sets of measured projections. Moreover, the reconstruction unit13 can be adapted to reconstruct the motion corrected three-dimensionalsecond computed tomography image based on the several sets oftransformations determined for the different measured projections groupsof the several sets of measured projections, the further transformationstransforming first computed tomography images of different sets ofmeasured projections such that they correspond to each and the measuredprojections.

For instance, if the radiation source 2 is rotated around the head alonga circular or helical trajectory over 360 degrees while measuring theprojections, the reconstruction unit 13 may divide the angular range of360 degrees into angular sub ranges of 20 degrees. Thus, a first set ofmeasured projections may correspond to an angular sub range from 0 to 20degrees, a second set of measured projections may correspond to anangular sub range from 21 to 40 degrees, a third set of measuredprojections may correspond to an angular sub range from 41 to 60degrees, et cetera. The reconstructing 13 can be adapted to reconstructfor each of these sets of measured projections, i.e. for each of theseangular ranges, a respective first computed tomography image. Thetransformation unit 14 may then be adapted to determine for each ofthese sets of measured projections, i.e. for each of these angularranges, transformations by using measured projection groups of therespective set of measured projection groups. For example, in order todetermine the transformations for the first angular range, a firstmeasured projection group corresponding to angular positions at 1 and 2degrees, a second measured projection group corresponding to angularpositions at 3 and 4 degrees, a third measured projection groupcorresponding to angular positions at 5 and 6 degrees, et cetera can beused. For each of these angular sub-sub ranges, i.e. a) 1 and 2 degrees,b) 3 and 4 degrees, c) 5 and 6 degrees, et cetera, a respectivetransformation can be determined, in order to determine for the angularsub range from 0 to 20 degrees a first set of transformations. Acorresponding set of transformations can be determined for each of theother angular sub ranges, i.e. for the second angular sub range from 20to 40 degrees, for the third angular sub range from 40 to 60 degrees, etcetera. Further transformations can be determined by registering thetransformed first computed tomography image, which has been determinedfor the angular positions at 19 and 20 degrees for the first set ofmeasured projections, with the transformed first computed tomographyimage which has been determined for the angular positions at 21 and 22degrees for the second set of measured projections. Correspondingly, afurther transformation can be determined by registering the transformedreconstructed first computed tomography images determined for theangular positions 39 and 40 degrees and 41 and 42 degrees, which belongto the second and third sets of measured projections, such that thesetransformed first computed tomography images correspond to each other.This determination of further transformations can be continued withfurther neighboring transformed first computed tomography images atborders of neighboring sets of measured projections, in order todetermine for the entire angular range of 360 degrees the furthertransformations. The reconstruction unit 13 can then be adapted to useall measured projections covering the 360 degrees together with the setsof transformations determined for the different sets of measuredprojections and the further transformations for reconstructing themotion-corrected second computed tomography image.

In the following an embodiment of a computed tomography image generationmethod for generating an image of a human head will exemplarily bedescribed with reference to a flowchart shown in FIG. 5.

In step 101 the radiation source 2 rotates around the rotational axis Rand the patient, especially the human head to be imaged, is moved alongthe rotational axis R such that the radiation source 2 moves relative tothe human head along a helical trajectory. In another embodiment theradiation source 2 and the human head can be moved relatively to eachother in another way, for instance, in a step-and-shoot acquisitionscheme. The radiation source 2 emits radiation traversing the human headand the detection device 6 detects the radiation, which has traversedthe human head, and generates measured projections based on the detectedradiation.

The measured projections are transferred to the projection processingdevice 18 and in step 102 the registration unit 13 reconstructs athree-dimensional first computed tomography image of the head based onthe measured projections. In step 103 the transformation determinationunit 14 determines three-dimensional transformations of the firstcomputed tomography image of the head for different measured projectiongroups, wherein in this embodiment each measured projection groupcomprises a single measured projection. The transformation determinationunit 14 determines for a certain measured projection group atransformation such that a degree of similarity between the certainmeasured projection group and a calculated projection group isincreased, especially optimized, wherein the calculated projection groupcorresponds to the certain measured projection group and is determinedby transforming the first computed tomography image in accordance withthe transformation to be determined and by forward projecting thetransformed first computed tomography image.

In step 104 the reconstruction unit 13 reconstructs a motion correctedthree-dimensional second computed tomography image based on the measuredprojections and the transformations determined for the differentmeasured projection groups, wherein in step 105 the reconstructed motioncorrected three-dimensional second computed tomography image is shown onthe display unit 11.

The image generation apparatus and the image generation method use asecond path motion estimation and compensation scheme to reduce motionartifacts during brain computed tomography scans, i.e. in reconstructedcomputed tomography images of a human head. As a prerequisite a firstthree-dimensional representation of the head is required, which isreconstructed from the available measured projections. Motion artifactsmay be partly reduced in this first three-dimensional representation,i.e. in the first computed tomography image, by using known motionreduction techniques like overscan weighting techniques. Thethree-dimensional representation of the human head is then forwardprojected for generating calculated projections, which are compared withthe measured projections, in order to estimate the three-dimensionalmotion of the human head, wherein the estimated three-dimensional motionis subsequently used in a motion compensated reconstruction scheme.

Thus, a three-dimensional head volume data set is reconstructed from themeasured projections, wherein the measured projections may be acquiredin a helical or step-and-shoot acquisition scheme. For each measuredprojection or for an angular subset of the measured projections, i.e.for each measured projection group, a forward projection of thereconstructed volume is preferentially calculated, in order to generatea calculated projection group which is compared to the respectivemeasured projection group using a similarity measure being, forinstance, a gradient angle difference, which may be normalized. In anembodiment the transformation determination unit, which can be regardedas being an optimizer, then determines the best three-dimensionaltransformation for the respective measured projection group to align thevolume with a single measured projection or with multiple measuredprojections with a small angular distance. For instance, a respectivemeasured projection group may comprise several measured projectionswhich correspond to an angular range of, for instance, 20 degrees orless, 10 degrees or less, or 5 degrees or less. The resultingtransformation, which is determined for the respective measuredprojection group, is stored as a motion representation for this measuredprojection group. After the transformations have been determined for thedifferent measured projection groups, an angular array oftransformations is present, i.e. transformations determined fordifferent measured projection groups and hence for different times ispresent, wherein these transformations may be filtered, interpolated oroutliers may be reduced. A motion compensated reconstruction may then beused to reconstruct an improved volume, wherein preferentially theinverse of the estimated motion is used to compensate the motion. Themethod may be applied as an iterative method.

In an embodiment the transformation determination unit is adapted todetermine whether the first computed tomography image comprises motionartifacts or not. If it is determined that the first computed tomographyimage does not comprise motion artifacts, the three-dimensionaltransformations are not determined and correspondingly a motioncorrected three-dimensional second computed tomography image is notreconstructed. For determining whether the first computed tomographyimage comprises motion artifacts or not, known motion artifactsdetection algorithms can be used. For instance, the transformationdetermination unit can be adapted to detect streaks originating fromedges in the first computed tomography image or other artifacts whichare known to be caused by motion. For detecting these artifacts in thefirst computed tomography image known segmentation algorithms can beused. The reconstruction unit can also be adapted to compare redundantmeasured projections, which correspond to opposing acquisitiondirections, after they have been rebinned into parallel ray geometry, inorder to determine whether motion has caused artifacts in the firstcomputed tomography image. If a difference between these parallelrebinned projections, which correspond to opposite acquisitiondirections, is smaller than a predefined threshold, it can be assumedthat no motion is present.

In an embodiment the reconstruction unit is adapted to, for generatingthe second image computed tomography image, perform the motioncorrection for a first part of the examination zone and to perform themotion correction not for a second part of the examination zone. Thefirst part preferentially includes the human head, whereas the secondpart may include the patient table.

Although in above described embodiments each measured projection groupand each calculated projection group comprises a single projection only,in other embodiments some or all measured projection groups andcalculated projection groups may comprise several projections. In anembodiment all measured projection groups and all calculated projectiongroups comprise a same number of projections. However, in anotherembodiment, the number of projections can be different for differentmeasured projection groups and different calculated projection groups.

Although in an above described embodiment a certain computed tomographysystem with a rotatable gantry and a radiation source and a detectiondevice mounted to the gantry has been described, in other embodimentsthe computed tomography image generation apparatus can be anotherapparatus for generating a computed tomography image. For instance, itcan be a C-arm computed tomography system which might have a flat paneldetector. Moreover, the projections providing unit may just be a storingunit which can provide stored, already measured projections or areceiving unit for receiving the measured projections and for providingthe received projections such that the computed tomography imagegeneration apparatus can be a computing device being adapted to generatea computed tomography image of the head.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality.

A single unit or devices may fulfill the functions of several itemsrecited in the claims. The mere fact that certain measures are recitedin mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage.

Procedures like the reconstruction of the first computed tomographyimage, the determination of the transformations, in particular, theforward projection procedures and the determination of the degree ofsimilarity, the reconstruction of the second computed tomography image,et cetera performed by one or several units or devices can be performedby any other number of units or devices. For example, steps 102 to 104can be performed by a single unit or by any other number of differentunits. The procedures and/or the control of the image generationapparatus in accordance with the image generation method can beimplemented as program code means of a computer program and/or asdedicated hardware.

A computer program may be stored/distributed on a suitable medium, suchas an optical storage medium or a solid-state medium, supplied togetherwith or as part of other hardware, but may also be distributed in otherforms, such as via the Internet or other wired or wirelesstelecommunication systems.

Any reference signs in the claims should not be construed as limitingthe scope.

The invention relates to a CT image generation apparatus for generatingan image of a head. Transformations of a first CT image of the head aredetermined for different measured projection groups, wherein for ameasured projection group a transformation is determined such that adegree of similarity between the measured projection group and acalculated projection group is increased, wherein the calculatedprojection group is calculated by transforming the first CT image inaccordance with the transformation to be determined and by forwardprojecting the transformed first CT image. A motion correctedthree-dimensional second CT image is reconstructed based on the measuredprojections and the transformations determined for the differentmeasured projection groups. This allows providing a high-quality CTimage of the head, even if a patient cannot stop moving the head in caseof, for instance, stroke.

1. A computed tomography image generation apparatus for generating animage of a human head, wherein the computed tomography image generationapparatus comprises: a projections providing unit for providing measuredtwo-dimensional projections of the head, wherein the measuredprojections have been measured at different times while a radiationsource, which emits radiation for traversing the head, has been movedaround the head and wherein the measured projections have been generatedbased on the radiation after having traversed the head, a reconstructionunit for reconstructing a three-dimensional first computed tomographyimage of the head based on the provided measured projections, atransformation determination unit for determining three-dimensionaltransformations of the first computed tomography image of the head fordifferent measured projection groups, wherein a measured projectiongroup comprises one or several measured projections, wherein thetransformation determination unit is adapted to determine for a certainmeasured projection group a transformation such that a degree ofsimilarity between the certain measured projection group and acalculated projection group is increased, wherein the calculatedprojection group corresponds to the certain measured projection groupand is calculated by transforming the first computed tomography image inaccordance with the transformation to be determined and by forwardprojecting the transformed first computed tomography image, wherein thereconstruction unit is adapted to reconstruct a motion correctedthree-dimensional second computed tomography image based on the measuredprojections and the transformations determined for the differentmeasured projection groups, wherein the computed tomography imagegeneration apparatus comprises an examination zone in which the humanhead is arrangeable, wherein the reconstruction unit is adapted toreconstruct the first computed tomography image and the second computedtomography image such that they show the examination zone and to, forgenerating the second computed tomography image, perform the motioncorrection for a first part of the examination zone and to perform themotion correction not for a second part of the examination zone, whereinthe first part of the examination zone includes the human head.
 2. Thecomputed tomography image generation apparatus as defined in claim 1,wherein the transformation determination unit is adapted to use agradient angle difference for determining the degree of similarity. 3.The computed tomography image generation apparatus as defined in claim1, wherein the reconstruction unit is adapted to reduce motion artifactsin the first computed tomography image by using overscan weighting. 4.The computed tomography image generation apparatus as defined in claim1, wherein the transformation determination unit is adapted to determinea transformation for a measured projection group iteratively.
 5. Thecomputed tomography image generation apparatus as defined in claim 1,wherein the transformation determination unit is adapted such that eachmeasured projection group comprises a single measured projection.
 6. Thecomputed tomography image generation apparatus as defined in claim 1,wherein the transformation determination unit is adapted such that eachmeasured projection group comprises several measured projections.
 7. Thecomputed tomography image generation apparatus as defined in claim 1,wherein the transformation determination unit is adapted to filter thedetermined transformations.
 8. The computed tomography image generationapparatus as defined in claim 1, wherein the transformationdetermination unit is adapted to determine transformations based on thedegree of similarity and the forward projection not for all times, atwhich projections have been measured, and to determine transformationsfor times, at which transformations have not been determined based onthe degree of similarity and the forward projection, by interpolation.9. The computed tomography image generation apparatus as defined inclaim 1, wherein the transformation determination unit is adapted todetermine an outlier in the determined transformations and to remove thedetermined outlier from the determined transformations.
 10. The computedtomography image generation apparatus as defined in claim 1, wherein:the reconstruction unit is adapted to divide the measured projectionsinto several sets of measured projections, wherein each set includestemporally adjacent measured projections, and to reconstruct severalthree-dimensional first computed tomography images of the head based onthe several sets of measured projections, wherein a respective firstcomputed tomography image is reconstructed based on a respective set ofmeasured projections, the transformation determination unit is adaptedto determine for each first computed tomography image a set ofthree-dimensional transformations of the respective first computedtomography image of the head for different measured projection groups,wherein a measured projection group comprises one or several measuredprojections of the respective set of measured projections, wherein thetransformation determination unit is adapted to determine for a certainmeasured projection group of the respective set of measured projectionsa transformation such that a degree of similarity between the certainmeasured projection group and a calculated projection group isincreased, wherein the calculated projection group corresponds to thecertain measured projection group and is calculated by transforming therespective first computed tomography image in accordance with thetransformation to be determined and by forward projecting thetransformed respective first computed tomography image, thereconstruction unit is adapted to reconstruct a motion correctedthree-dimensional second computed tomography image based on the measuredprojections and the several sets of transformations determined for thedifferent measured projection groups of the several sets of measuredprojections.
 11. The computed tomography image generation apparatus asdefined in claim 10, wherein the transformation determination unit isadapted to determine a further transformation transforming a transformedfirst computed tomography image, which has been reconstructed based on aset of measured projections and which has then be transformed, such thatit corresponds to another transformed first computed tomography image,which has been reconstructed based on another set of measuredprojections and which has then be transformed, wherein thereconstruction unit is adapted to reconstruct the motion correctedthree-dimensional second computed tomography image also based on thefurther transformation.
 12. The computed tomography image generationapparatus as defined in claim 10, wherein the reconstruction unit isadapted to divide the measured projections into several sets of measuredprojections such that at least one set of measured projections does notfulfill the completeness criterion for computed tomographyreconstruction.
 13. The computed tomography image generation apparatusas defined in claim 1, wherein the reconstruction unit and thetransformation determination unit are adapted such that, after thesecond computed tomography image has been reconstructed, in an iterationstep the transformations are determined again based on the secondcomputed tomography image, which then has the function of the firstcomputed tomography image, and the second computed tomography image isagain reconstructed based on the newly determined transformations andthe measured projections.
 14. A computed tomography image generationmethod for generating an image of a human head, wherein the computedtomography image generation method comprises: providing measuredtwo-dimensional projections of the head by a projections providing unit,wherein the measured projections have been measured at different timeswhile a radiation source, which emits radiation for traversing the head,has been moved around the head and wherein the measured projections havebeen generated based on the radiation after having traversed the head,reconstructing a three-dimensional first computed tomography image ofthe head based on the provided measured projections by a reconstructionunit, determining three-dimensional transformations of the firstcomputed tomography image of the head for different measured projectiongroups by a transformation determination unit, wherein a measuredprojection group comprises one or several measured projections, whereinthe transformation determination unit determines for a certain measuredprojection group a transformation such that a degree of similaritybetween the certain measured projection group and a calculatedprojection group is increased, wherein the calculated projection groupcorresponds to the certain measured projection group and is calculatedby transforming the first computed tomography image in accordance withthe transformation to be determined and by forward projecting thetransformed first computed tomography image, wherein the reconstructionunit reconstructs a motion corrected three-dimensional second computedtomography image based on the measured projections and thetransformations determined for the different measured projection groups,wherein the computed tomography image generation apparatus comprises anexamination zone in which the human head is arrangeable, wherein thereconstruction unit reconstructs the first computed tomography image andthe second computed tomography image such that they show the examinationzone and to, for generating the second computed tomography image,perform the motion correction for a first part of the examination zoneand to perform the motion correction not for a second part of theexamination zone, wherein the first part of the examination zoneincludes the human head.
 15. A computer program for controlling acomputed tomography image generation apparatus, the computer programcomprising program code means for causing the computed tomography imagegeneration apparatus to carry out the steps of the computed tomographyimage generation method as defined in claim 14, when the computerprogram is run on a computer controlling the computed tomography imagegeneration apparatus.